This invention involves the application of phased array techniques to millimeter wave imaging. Specifically, the invention involves focal plane and pupil plane array imaging techniques using reconfigurable phased antenna arrays.
Millimeter-wave imaging involves passive detection of naturally occurring radiation in the millimeter wave (30-300 GHz) band. There are also active millimeter wave imaging systems, which illuminate the target with millimeter wave radiation. The techniques described here are also applicable to the receiver portion of active imagers. However, passive imagers have the advantage of no millimeter wave emissions, making their use difficult to detect and eliminating perceived health issues with millimeter wave radio emissions.
Atmospheric propagation windows for millimeter wave radiation (in which there is minimal atmospheric absorption of the radiation) exist at 35, 94, 140, 220 GHz, and thus, many millimeter wave imagers are designed to operate at these frequencies. However, imagers are also designed to operate at other frequencies, particularly in cases where detection of radiation is required only over relatively short distances (e.g., 10 m). Millimeter wave imagers are able to image in low-visibility conditions (as opposed to visual/infrared imagers), and millimeter wave imagers are particularly useful for imaging objects through fog or dust. They are also useful in security applications because they have ability to detect objects through clothing.
Millimeter wave imagers are most typically built using a system of millimeter wave lenses to focus millimeter wave radiation on one or more detectors. The detectors often consist of millimeter wave radio receivers and antenna elements. An image of many pixels can be created by mechanically scanning radiation from different portions of the scene sequentially onto a single detector element. Alternatively, there may be multiple detectors, arranged as a linear array or as a focal plane array. Using multiple detectors increases the dwell time of a particular detector on a single pixel of the scene. This increased integration time reduces the effective noise floor of the detector and improves the thermal resolution of the imager.